1. Field of the Invention
The present invention relates to high strength spring steel that is used as the material of, for example, suspension springs, torsion bars and stabilizers for automobiles, and in particular, to high strength spring steel that possesses high strength as well as excellent pitting corrosion resistance and corrosion fatigue property and that is preferably used as chassis underbody members of automobiles, and a method for manufacturing the same.
2. Description of Related Art
From the viewpoint of recent global environmental issues, there are demands for improving fuel efficiency of automobiles and reducing carbon dioxide emission, and therefore an increasingly high demand for reducing weight of automobiles. Particularly, there is a strong demand for reducing weight of suspension springs that are chassis underbody members of automobiles, whereby high stress design is applied to these suspension springs by using as a material thereof a strengthened material having a post quenching-tempering strength of 2000 MPa or more.
General-purpose spring steel has a post quenching-tempering strength of about 1600 to 1800 MPa, as prescribed in JIS G4801. Such spring steel is manufactured into a predetermined wire rod by hot rolling and the wire rod is thermally formed into a spring-like shape and subjected to quenching-tempering processes in a case of a hot formed spring. Alternatively, the spring steel is subjected to drawing, quenching-tempering processes and then formed into a spring-like shape in a case of a cold formed spring.
For example, the materials commonly used for suspension springs include SUP7 described in JIS G4801. When SUP7 is strengthened, corrosion fatigue property (corrosion fatigue resistance) thereof after corrosion deteriorate, although fatigue properties thereof in the atmosphere improve, thereby eventually causing a problem of deterioration in corrosion fatigue property. In view of this, the current upper limit of actually applicable hardness of SUP7 is a level of 51 HRC and the upper limit of design stress thereof is 1100 MPa, inhibiting further enhancement of SUPT strength.
A material that is strengthened so as to have a strength of 1900 MPa or more after quenching-tempering processes has higher crack sensitivity. Accordingly, if a component that is exposed to the exterior, such as a suspension spring as a chassis underbody member of an automobile, is made of such a material as described above and has poor pitting corrosion resistance, there is a concern that corrosion pits may be formed at those portions where the coating has come off due to pebbles and that the chassis underbody member may be damaged due to the propagation of fatigue cracks starting from the corrosion pits.
In view of the foregoing, some solutions have been proposed to address these problems. JP-B-2932943 discloses that, by controlling the chemical composition and the value of FP (see Formula (1a) below) to be between 2.5 and 4.5, no supercooling structure occurs in the structure after rolling, the strength after rolling is suppressed to be 1350 MPa or less at which cold working is facilitated, and uniform and sufficient hardening is obtained by the subsequent quenching and tempering, which makes it possible to obtain the strength after quenching and tempering being 1900 MPa or more. However, JP-B-2932943 is based on the addition of an alloy element for improving corrosion resistance and controllably setting the value of FP to be between 2.5 and 4.5 does not necessarily ensure provision of a high strength spring steel that possesses good pitting corrosion resistance and corrosion fatigue property.FP=(0.23[C]+0.1)×(0.7[Si]+1)×(3.5[Mn]+1)×(2.2[Cr]+1)×(0.4[Ni]+1)×(3[Mo]+1)   Formula (1a)wherein [brackets] denote the content of each element in the brackets (in mass %).
JP-A 10-196697 discloses spring steel that is obtained by covering at least a part of the surface of a spring steel base material with a corrosion protective film functioning as a sacrifice anode, wherein carbonitride forming elements are added to the spring steel base material so that carbonitride is micro-dispersed in the spring steel basic material. In JP-A 10-196697, there is used as a corrosion protective film either a metal film that is composed of metal/alloy having electrochemically lower potential than the spring steel basic material or a composite film in which many metals/alloys of the metals having electrochemically lower potential than the spring steel basic material are dispersed in a non-metal film. This, however, leads to an increase in manufacturing cost due to the need for performing a step of forming a corrosion protective film on the spring steel. It is also believed that if the corrosion protective film comes off due to pebbles and the like, corrosion pits are formed and deteriorate corrosion fatigue property.
JP-B 3896902 discloses that C is to be reduced as the cause of a reduction in corrosion fatigue strength, that degradation in sag resistance that could be caused by the reduction in C is prevented by adding Si, and that the ratio of Si/C is important in this regard. However, there is a limit to reducing the amount of C, if reduction of carbon content effectively suppresses deterioration in the corrosion fatigue strength. Thus, simply setting a ratio of Si/C alone does not necessarily provide high strength spring steel that possesses both good pitting corrosion resistance and corrosion fatigue property.
Patent JP-B 4280123 discloses that reducing the content of Cr may suppress the amount of hydrogen generated at the tip of corrosion pits, therefore the amount of hydrogen penetrating into the steel and eventually the degree of hydrogen embrittlement. Patent JP-B 4280123 also discloses that if any hydrogen penetrates into the steel material, the degree of hydrogen embrittlement may be suppressed by trapping hydrogen by Ti and V, and therefore the corrosion fatigue resistance may be improved by balancing the contents of Cr, Ti and V appropriately. However, even if the degree of hydrogen embrittlement of the spring steel can be suppressed by only optimizing the contents of Cr, Ti and V, high strength spring steel that possesses good pitting corrosion resistance and corrosion fatigue property may not necessarily be obtained by such optimization.
JP-A 2008-106365 discloses that corrosion fatigue property may be improved by subjecting the steel to heat treatment to have a hardness of 50.5 to 55.0 HRC, followed by warm shot peening so that a residual stress of 600 MPa or more is generated at a depth of 0.2 mm below the surface. This, however, leads to an increase in manufacturing cost due to the need for performing a step of shot peening the spring steel. Further, while the provision of residual stress by shot peening is effective for suppressing the occurrence of surface cracks, it does not necessarily provide high strength spring steel that possesses both good pitting corrosion resistance and corrosion fatigue property.
JP-A 2009-046764 discloses spring steel that has excellent corrosion fatigue property by balancing appropriately the contents of C, Si, Mn, Cr, Ni and Cu from the viewpoint of the hardness of spring steel, the amounts of C, Cr, Ni and Cu from the viewpoint of the shape of pits, and the amounts of C, Si, Mn, Cr, Ni, Cu, Ti and Nb from the viewpoint of hydrogen embrittlement resistance. However, there is a limit on optimizing the shape of pits only by balancing the amounts of C, Cr, Ni and Cu.